The invention relates to an illumination system for a colour image projection device, successively comprising a radiation source for supplying a radiation beam and at least a cholesteric mirror for splitting the radiation beam into at least two sub-beams having a different wavelength.
The invention also relates to a circular polarizer suitable for use in such an illumination system for directly converting unpolarized radiation into circularly polarized sub-beams.
The invention further relates to a colour image projection device comprising such an illumination system and a circular polarizer.
The term colour image projection device should be considered to have a wide meaning and may be used for a device for displaying a video image, a graphic image, numerical information or a combination thereof.
An illumination system suitable for use in a colour image projection device of the type described in the opening paragraph is known from the article "New Liquid Crystal Polarized Color Projection Principle" by M. Schadt and J. Funfschilling in Japanese Journal of Applied Physics, vol. 29, no. 10, October 1990, pp. 1974-1984. The image display panels of the illumination system used in the known device are transmission panels having a layer of liquid crystalline material as an image-forming element. This layer modulates the state of polarization of incident radiation in conformity with the image information present therein. To this end the beam supplied by the illumination system should be linearly polarized in a given direction or should have a given direction of polarization rotation, dependent on whether the image display panel is suitable for modulating linearly or circularly polarized radiation. An image display panel is herein understood to mean the combination of the liquid crystalline layer with a polarizer and an analyser.
The illumination system described in the article makes use of cholesteric mirrors for splitting the "white" unpolarized radiation beam emitted by the radiation source into a plurality of "coloured" sub-beams in conformity with the number of image display panels and in a defined state of polarization. Thus, the cholesteric mirrors have a dual function, viz. colour selection and polarization selection.
A cholesteric mirror has an optical layer of liquid crystalline material with a spiral or helix-like structure having a pitch p. When a "white", unpolarized radiation beam is incident on such a mirror, a circularly polarized radiation component having a direction of rotation corresponding to the direction of rotation of the molecular helix and a wavelength adapted to the pitch p of the helix will be reflected, while a component having the opposite direction of rotation and/or a wavelength not adapted to the mirror will be passed.
In the known illumination system the colour separation and polarization is effected as follows. Unpolarized, white light is incident on a first cholesteric mirror which is oriented at an angle of 45.degree. with respect to the beam. The blue, laevorotatory circularly polarized sub-beam is reflected towards a plane mirror. The direction of rotation is inverted on the mirror into a dextrorotatary direction so that the direction of rotation of this sub-beam is no longer adapted to the helix of the cholesteric mirror and consequently the sub-beam will be passed in the direction of the blue image display panel. The rest of the radiation beam is passed to a second cholesteric mirror which selects the blue, dextrorotatary circularly polarized radiation component and directly reflects it in the direction of the blue image display panel. Analogously, the green and the red sub-beam are selected by means of further cholesteric mirrors.
A drawback of the known illumination system is that the selectivity for laevorotatory and dextrorotatary circularly polarized radiation is not always sufficiently large, so that the efficiency of the conversion of unpolarized radiation into circularly polarized radiation is not optimal.